Radiochemistry Volume 3

A Review of the Literature Published During 1974 and 1975

By G. W. A. Newton

The Royal Society of Chemistry

Copyright © 1976 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-274-3

Contents

Chapter 1 Industrial Applications of Radioisotopes By J. A. Heslop,
Chapter 2 Activation Analysis in Archaeology By G. Harbottle,
Chapter 3 Preparation of Radiopharmaceuticals and Labelled Compounds using Short-lived Radionuclides By D. J. Silvester,
Chapter 4 Sample Preparation Procedures for Liquid Scintillation Counting By B. W. Fox,
Author Index, 133,

CHAPTER 1

Industrial Applications of Radioisotopes

BY J. A. HESLOP

The application of radioisotopes to the study of industrial process and to the measurement and control of those processes is well established. This review will contain little that is radically novel in terms of basic techniques or applications. There exists, however, a basic information gap between the radioisotope applications specialist and the user, be he chemical engineer, R and D scientist, or civil engineer. This gap causes the use of radioisotopes as tracers or in instruments to be considered as a last-gasp effort that is to be used only when all else fails. Experience has shown that the use of radioisotopes can often solve problems much more easily than conventional techniques, and, in cases where the radioisotope specialists are part of the industry or belong to an institute with good industrial contacts, the number of radioisotope applications can show an amazing growth (e.g. the group at TCI Petrochemicals Division who are concerned with the application of radioisotopes within industry complete well in excess of 1000 applications per year). It is hoped that this review will help to bridge the information gap in that it covers recent applications of radioisotopes in industry up to about mid-1975.

The field has already been the subject of a number of general reviews which cover the basic principles and certain applications. Specific reviews covering applications in the textile and fibres industry, in plastics, and in the basic metals industry have been published.

1 Radiotracers

It has been said that there is no such thing as the perfect tracer, as this material would be indistinguishable from the population as a whole. Isotopes usually provide the nearest approach to the perfect tracer, in that they are, to a good first approximation, chemically and physically indistinguishable from the total population. If they are also radioactive then they possess a useful property (the emission of radiation) which can be related to their concentration in the bulk of the medium under investigation. There are several degrees of sophistication possible in the selection of a tracer. If the medium under investigation undergoes a chemical reaction or a phase change, then a tracer which parallels this behaviour must be selected, and hence the chemical form of the tracer must be identical with the bulk material. If this criterion can be satisfied, then, neglecting any possible isotope effects, the ideal tracer will be used.

In the chemical industry, the isotopes which come into this category are usually 14C and 3H, both of which can be readily incorporated into organic materials. These isotopes are far from ideal for use in industry, in that they are both weak β-emitters and hence cannot easily be detected inside process equipment, and sampling is necessary. They both have long half-lives and hence cannot easily be used to study problems in which they would end up in a product which would be sold to the general public. Despite these disadvantages, there are certain problems which can only be solved by the use of 14C or 3H, and, provided the financial incentive is large enough, both isotopes can be used on full-scale plant both efficiently and safely. On smaller scale equipment and in the laboratory 14C and 3H play very important parts in industrial monitoring.

In a majority of situations, where the bulk property of a material is being followed, there are no physical or chemical changes, and it is possible to use a physical tracer. This type of tracer will follow the bulk movement of a material, provided that it does not undergo a phase change or a chemical reaction. Similarly, the tracer itself must not be precipitated or in any way lost from the material being traced. It is often sufficient if the material is simply soluble in the medium, as in the use of NH482Br for the tracing of aqueous streams. The use of a physical tracer allows a γ-radiation detector to be used outside the process vessel, and hence removes the need for samples to be taken. This means that short-lived isotopes can be used, thus reducing the problems of radiological protection. Table l is a list of a number of isotopes which have been used in the study of various industrial problems. The isotopes are in general produced in a nuclear reactor and are of sufficiently long half-life to allow easy transport to the industrial site. Exceptions to this are 41A and 56Mn, whose half-life requires reasonably rapid access to an isotope production facility. The production of isotope generators for use in medicine has not been widely applied in industry, usually because the γ-energy requirements are different, although a 140Ba/140La generator has been successfully developed and used by R u nge and Grahl to investigate matter-transport processes in chemical plants.

Flow Measurements. — Most industrial processes require measurements of mass flow in order that the efficiency of their operations can be monitored. A large number of conventional flowmeters exist but they often need calibration, or the measurement of an unmetered flow is required. Radioisotope methods have achieved a wide acceptance, often as standard methods of flow measurements where high accuracy is required, for example in the measurement of a mass balance on a plant.

There are several radiotracer methods of flow measurement in common use. The simplest is the pulse-velocity measurement, in which a sharp pulse of radioactivity is injected into the stream whose flow is to be measured. The passage of the pulse is observed by a pair of detectors positioned downstream of the injection pipe at a distance such that lateral mixing within the pipe is complete. The flow rate is given by Q = lA/t, where Q is the volume flow rate, A the cross-sectional area, l is the distance between the two detectors and t is the time taken for the radioactivity to travel the distance l. If a γ-emitting isotope is used then the detectors can be outside the pipe and can be connected to either simple or integrating ratemeters. I n both cases the time t is obtained by measuring the time interval between the peak half-height positions. The method requires turbulent flow in the pipe and a knowledge of the internal pipe diameter, which must be effectively constant. This latter measurement can be obtained by measuring the external diameter of the pipe and then the pipe wall thick ness, using, for example, an ultrasonic method. If accurate flow rates are required this is done at several points along the pipe length, and measurements are also made of the pressure (for gases) and temperature within the pipe.

Under ideal conditions, Evans et al. measured the flow rate of air along a pipe, using 85Kr at flow rates varying from 3 to 300 I s-1. Over 90% of a total of 61 tests carried out had a mean deviation of <± 0.4% from the flow rate determined by collection of the gas over a given time interval.

Under industrial conditions it is difficult to achieve this sort of accuracy, but deviations of [+ or -] 1 – 2% are usually easily achievable, and under conditions in which the pulse-velocity technique can be used it will be the method of choice for flow measurement.

Flows of liquids and gases can also be measured by a dilution techn ique in which a radioactive tracer (radioactive concentration S1) is continuously injected at a rate q into the material flow. Samples are taken at a suitable distance from the injection point. If S2 is the radioactive concentration at the sample point and Q is the flow rate to be measured, then

qS 1 = (Q + q) S2 (1)

Q = q (S 1 –S2)/S2 (2)

In general, S1 [??] S2 [therefore]

Q ≈ q S1/S2 (3)

Provided that turbulent flow exists, the measurement of flow rate is independent of the vessel diameter and any changes in it, and hence the method can be used in situations where the pulse-velocity method cannot be applied. Injection is continued until a radioactivity ‘plateau’ is obtained at the sampling point, and this can be measured with good precision. The radioactive concentration of the injected material and the rate of injection can be accurately measured, and hence overall accuracies of [+ or -] 1% can be attained.

The accuracy of the method has been tested against direct weighing by measuring the flow rate along a pipe to a road tanker by the dilution method and comparing this with the weight of material found in the tanker. The results shown in Table 2 show the excellent agreement obtained between the direct method and the dilution technique. This method has been applied by Clayton and Evans to the measurement of flow through turbines and pumps in power stations and is in routine use throughout ICI for the measurement of a wide variety of gas and liquid flow rates. Gas flow rates in excess of l05 m3 h-1 and liquid flow rates greater than 106 gallon h-1 have been measured.

A third method of flow measurement exists which is useful for the measurement of large flows in open channels. The dilution sudden-injection method has a number of variations but essentially consists of the injection of a suitable tracer for a short duration, followed by downstream sampling over a period of time sufficiently long to ensure that the whole of the tracer has passed the sampling point.

Then

[MATHEMATICAL EXPRESSION OMITTED] (4)

giving Q= S1VF/N (Total count method) or Q = S1VF/rt (Continuous Sample method), where F is the efficiency of the counting set-up, S1 is the specific activity of the injection solution, S2 are the specific activities of the samples, Q is the volume flow rate, t is the time of sampling after tracer injection, N is the total number of counts accumulated, and [bar.r is the counting rate for a homogenized sample. The flow rate can be determined either by sampling (total sample method) or simply by using a ratemeter and obtaining a count versus time trace at the sampling position (total count method). Neither method is as accurate as the dilution flow, but they are convenient in the measurement of large flows in open channels, e.g. effluent flow, as the amount of radioactivity required is much less than that used for the dilution flow method.

Radioisotope measurements can provide an instantaneous method for the measurement of flow with an ease and accuracy that is difficult to achieve by other methods. The technique is widely used within the oil and chemical industries, both for the measurement of unmetered flows and for the calibration of existing flow-meters. The checking of material balances within chemical plants usually depends ultimately on the accuracy of measurement of the flow rates of the materials entering and leaving the plant. In recent years the greatly increased legislation concerned with environmental conditions has led to a greatly increased demand for the measurement of flow in drains and open channels.

Leak Detection. — The ease of detection of radioactivity and the unambiguous nature of the qualitative determination of its presence make the use of radiotracers ideal for the detection and measurement of leaks. The methods used depend on the system under investigation but the techniques have been applied in situations ranging from underground pipelines to heat-exchanger shells, reactor cooling systems, and aircraft tyres. Leaks are often detected by the continuous injection of a radioactive tracer until the section within which the leak is suspected is uniformly la belled. The tracer within the pipe is then flushed away and a survey is conducted to detect any residual activity in the area adjacent to the pipeline or vessel which has escaped through leakage. It is often advantageous if, before flushing the pipeline to remove radioactivity, a valve is closed and the pipeline is pressurized. Detection of the residual activity from the leak may be carried out by th e use of sensitive detectors by traversing the pipeline length or by the use of a pig containing a radioactivity detector which traverses the line within the pipe and prints out the distance from the starting point whenever the radioactivity exceeds a predetermined threshold.

In chemical plants, which often involve a number of process units and heat exchangers, often the first indication of leakage problems is the production of off-specification material. It is obviously very desirable to isolate any leakage, so that the plant down-time may be minimized or avoided by the use of alternative equipment. In situations like this the leaks must be detected while the plant is on-line, and the technique generally used is the injection of a pulse of activity into the material (often cooling water) that is thought to be leaking followed by sampling of the product. This is carried out at a number of points until the position of the leakage is known to be within a single u nit. Physical tracers can often be used for this type of study, e.g. 24Na for the detection of leaks of cooling water, but care must be exercised in some cases where leakage may take place from the liquid or the gas phases, e.g. leaks of steam into process material, where negative results have been obtained using both a liquid (24Na) and gaseous (41A) tracer but a significant leak has been detected when a chemical tracer is used (3H2O). In this case it is obviously necessary that the tracer be compatible with both systems, i.e. the leaking material and the product material, a condition which is difficult to achieve with physical tracers.

Material Movement and Residence-time Distributions. — The way in which material moves through a processing unit must obviously have a profound effect on the quality of the final product obtained. Thus a study of the mode of matter transport within a given unit may greatly influence the mode of operation of that unit and also the design of any new units.

The simplest application of radioisotope techniques in this type of study is in the measurement of mixing times in a simple batch process. In this case a pulse of radioactivity is introduced into the mixing vessel and then samples are taken at intervals until the distribution of radioactivity within the system is uniform. The determination of mixing times obviously allows the most effective use to be made of expensive mixing equipment.

Residence-time Distributions. Applications of tracers in the assessment of mixing are trivial compared with the information that can be obtained by injection of a pulse of material into the process stream followed by monitoring of the pulse at a later stage in the process. The shape of the pulse may be analysed so that information is obtained about the movement of material in this part of the process.

In the chemical industry one frequently used model of the process system is that of stirred tanks or pots. The exit pulse is analysed in terms of the mean residence time tm) and the number of perfectly stirred pots (n) in a cascade which would produce the observed exit trace. For a closed system into which a pulse of radioactivity is injected (Δ function) the shape of the exit trace can be expressed as:

[MATHEMATICAL EXPRESSION OMITTED] (5)

where Γ(n) = Gamma function, t1 = delay time, t= real time.

This simple analysis can be used where the data obtained are fairly crude. With modern injection and detectors the data are usually good enough to allow further analysis in terms of the moments of the residence-time probability density, E, of the tracer, derived from the moments of the input and out put traces. In this analysis the input trace is not necessarily a Δ function. The various moments can be analysed for various situations, e.g. open and closed pipes with axial diffusion, closed apparatus with gamma distribution and delay, and adsorption processes. The analysis applied to the output curves depends on the information required, and various attempts have been made to gain detailed information from the tracers. Niemi used a particulate 59Fe tracer to study flotation processes in the metallurgical industry. The exit pulse is analysed by the method of least squares. An ideal injection impulse is assumed and the results are used to predict the behaviour of particles under varying conditions of process loading.

A simpler approach has been adopted in the determination of the residence times and the degree of mixing of coke and limestone in iron furnaces. In the study of material movement of solids the choice of tracer is very important, as the behaviour of the material can depend on a number of parameters, e.g. density, particle size, and particle strength. The value of using reactor-irradiated process material has been demonstrated by a study on a ferrochrome smelter, where the use of activated process material revealed that the pellets of chrome concentrate, the lump ore, the coke, the dolomite, and the quartz feeds to the preheater kiln all had different residence times, depending on the grain size of the particles. The knowledge of the residence time allowed several process developments to be carried out, resulting in a decrease in the amount of unusable product.

(Continues…)Excerpted from Radiochemistry Volume 3 by G. W. A. Newton. Copyright © 1976 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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